In U.S. Pat. No. 3,842,194, issued to John K. Clemens, on Oct. 15, 1974, and assigned to RCA Corporation, a video disc playback system of a variable capacitance form is disclosed. In one embodiment described therein, a capacitive pickup is utilized to read signals from the surface of an information record, typically a plastic disc similar to an audio disc, that contains stored video and audio information. The information record in the above embodiment has a fine spiral groove to guide a stylus that contains a thin electrode (about 0.2 micrometer thick), and the video and audio information is stored in the form of depressions in the bottom of the groove. The groove is typically about 2.7 micrometers in width and about 0.5 micrometer in depth. For the signal to be detected capacitively with a high quality, the information record itself is conducting or has a conductive layer on it. The stored information is read by measuring the capacitive variations between the electrode on the stylus and the surface of the record. Detection of the small capacitive variations is performed by forming a resonant circuit with the stylus-record capacitance. Pickups made in this manner can resolve signal elements signficantly smaller than the wavelength of light with sufficient reliability to reproduce high quality video and audio signals.
In systems of the above type, the stylus has a tip at one end thereof adapted to track a path along the surface of the information record and to measure capacitive variations between the stylus and the surface of the record when relative motion is established between the stylus and the record. In embodiments where the stylus tracks a groove and supports an electrode that is coated on its trailing face, the stylus may be made of either sapphire or diamond. The shape of the stylus tip forms a footprint. Many shapes are possible, and the footprint dimensions are quite variable, although extremely small. Typically, the stylus tip has a prow or front portion terminating at a base thereof, and a stern or back portion remote from the prow. The prow and stern, along with a pair of side surfaces, usually define the shape of the bottom portion (footprint), with the shoe length of the stylus being the distance between the base and the stern along a line tangent to the path at the tracking location.
In the video disc playback system, the shoe length of the stylus is about 3 to 4 micrometers. In the mass production of video disc playback styli there is a need for rapid measurements of stylus shoe length. Such a measurement system must be suitable for controlling or measuring the stylus shoe length during high volume production or testing of the styli. This is particularly true in the final lapping process wherein a V-shaped lap is formed in the bottom portion of the stylus tip. The present invention provides a novel system for rapidly measuring stylus shoe length during a manufacturing operation.